What is Simulation and Optimization in Water Resource Engineering ?

Simulation 

Simulation is the process of designing a model of a system and conducting experiments with it for understanding the behavior of the system and for evaluating the various strategies for its operation. A simulation model can be physical, analog or mathematical. The vehicle used to solve the mathematical model is commonly a computer. While performing a simulation using computers, the first step is to prepare a simulation model within a specific computer software. The simulation model should exactly replicate the real world problem. Then, known input parameters are fed to the simulation software as boundary conditions. And, the final step includes running the simulation model using the simulation software in which fundamental mathematical equations that govern the given problem are solved using computer resources. Different techniques like finite element method (FEM), finite difference method (FDM), finite volume method (FVM) etc are used to solve the fundamental governing equations. The advantages of computer simulation over a physical modelling are as follows:

1. A physical model is a time consuming and costly affair.
2. Complexities may arise in physical modeling if it is required to evaluate the alternative configurations and sizes of the facility.

Optimization

Optimization is the process of finding a best solution from a number of possible alternatives. The optimization method finds a set of decision variable such that the objective function is optimized. Optimization techniques are used to find optimal or near-optimal solutions to complex problems where exhaustive search is not practical. The objective could be to minimize cost, maximize profit, proper utilization of resources etc.

Applications of Simulation and Optimization Techniques in Water Resources Engineering 

There are various simulations software available free of cost or in paid versions that can be used to simulate the problems in water resources engineering. And analyzing the results of such simulations, optimal solutions can be obtained. For example, simulation of water flow in rivers can be performed in HecRAS in order to understand the flow hydraulics in natural streams, simulation of water hammer effect can be performed in a software named Hammer, three dimensional sediment laden flow simulation can be performed in Flow3D, the rock fall simulation can be performed in a software called RocFall by rocscience.

Anchor Block & Support Pier in Hydropower Plant

Anchor Block / Thrust Block

Anchor block is an encasement of penstock pipe at particular section, designed to restrain the pipe movement in all direction. It is a massive concrete block that anchor down the pipe securely to the ground. It should be stable against various forces acting on it. The shape and size of anchor block is confirmed by stability analysis. An expansion joint in the pipe is placed immediately downstream of the anchor block. Anchor block is required at following locations along the pipe line:

1. At every horizonal and vertical bends. Due to change in direction of flow, huge hydrostatic force acts at pipe bends that tend to move the pipe out of the alignment which is resisted by the anchor block.
2. At immediately upstream of the powerhouse. This minimizes the stresses in turbine housing.
3. In straight section at an interval of 100 to 150 m. This interval may become considerably small for micro hydel plants where the total head and discharge is relatively low.
4. At bifurcation and trifurcation in pipeline system. The flow redistribution at such branching creates imbalances in flow rates, pressure and velocities leading to uneven forces which is resisted by the anchor blocks.

Saddle Support / Support Pier

Support piers are short columns that are placed between anchor blocks along straight sections of exposed penstock pipe. Supports piers or saddle supports are provided at uniform spacing along the pipeline. These structures prevent the pipe from sagging and becoming overstressed. However, support piers allow pipe movement parallel to pipe alignment that occurs due to thermal expansion and construction. 

Mandatory Rule of Thumb in Building Design

In building design for Nepal, there are several mandatory rule of thumb that architects and designers typically follow. While these rules may or may not be legally binding, they are based on experience and practical knowledge to ensure safe and functional buildings. The main objective of these Mandatory Rules of Thumb (MRT) is to provide ready-to-use dimensions and details for various structural and non-structural elements for ordinary residential buildings commonly built in Nepal. Their purpose is to replace the non-engineered construction presently adopted with pre-engineered construction so as to achieve the minimum seismic safety requirements. There are major three codes that explain the thumb rule for building design in Nepal. They are NBC 201, NBC 202 and NBC 205. NBC 201 is for RCC building with masonry infill. NBC 202 is for load bearing masonry building and NBC 205 talks about MRT for RCC building without masonry infill. Few important rule of thumbs for building design as per these codes are pointed below:

For Load Bearing Masonry Building

• Brick masonry with cement mortar can be used upto three storey building.
• Brick masonry with mud mortar can be used upto two storey.
• Stone masonry with cement mortar can be used upto two storey.
• The concrete to be used in footings, columns, beams and slabs, etc., shall have a minimum crushing strength of 15 kN/m² at 28 days for a 150 mm cube.
• Cement-sand  mortar for masonry bond shall be of 1:6 and 1:4 for one-brick and half-brick thick walls, respectively.
• All plasters shall have a cement-sand mix not leaner than 1:6 on outside or inside faces.
• In order to achieve the full strength of masonry, the usual bonds specified for masonry shall be followed so that the vertical joints are broken properly from course to course.
• Openings should be as small and as centrally located as practicable.
• A building shall not be constructed if the proposed site is : water-logged, a rock-falling area, a landslide-prone area, a subsidence and/or fill area, and, a river bed or swamp area.

For RCC Building

• The span of beam shall not exceed 4.5m
• Each slab panels must be lesser than 13.5 square meter.
• The size of cantilever projection shall not exceed 1m.
• The length to width as well as height to width ratio of building must not exceed 3.
• The maximum height of structure is 11m or 3 storey whichever is less from the level of lateral restraint. However, an additional storey of smaller plan area(not exceeding 25% of typical floor area) shall be permitted.
• The length of wings on the structure shall restricted such that they are lesser than 25% of the length of rectangular part in either direction.
• No walls except a parapet wall shall be built on a cantilever slab. Such walls shall be constructed only if the cantilever slab is formed with beams.
• The foundation shall be at uniform level.
• Buildings shall not have soft storey.

Factors Affecting Selection of Turbine

Hydropower projects utilize various types of turbines depending on the site conditions and project requirements. The major types of turbines installed in the powerhouse of a hydropower project are Pelton turbine, Francis turbine, Propeller and Kaplan turbine, Deriaz turbine etc. Among them, Francis and Pelton turbine are the most widely used turbines in hydropower production because they have been extensively studied, developed and optimized over the years. The selection of a turbine for a hydropower project is a crucial decision that depends on various factors which are explained below:

1. Head and Discharge

As a general rule, Francis turbine is mostly suitable for high discharge and low head applications where as Pelton turbine used in case of high head and low discharge condition.

2. Part Load Operation

Efficiency of turbine is maximum when it is running at design load condition. In case of part load operation, Pelton turbine proves to be more efficient than Francis turbine.

3. Rotational Speed

As the turbine and generators are directly coupled, the rotational speed of turbine is same as synchronous speed of generator and is given as:

$$N=\frac{120f}{N_p}$$

where,

N = Rotational speed, rpm

$N_p$=Number of poles

f = Electrical frequency, Hz (50Hz for Nepal)

As the rotational speed (N) increases, number of pole ($N_p$) required is less. This means size of generator is reduced which ultimately reduces the cost of construction of powerhouse.

4. Specific Speed

The specific speed relation can be written as:

$$N_s=\frac{N\sqrt{P_{HP}}}{H^{5/4}}$$

$\because N_s \propto N$

The turbine with higher specific speed is expected to have high rotational speed. Eg. Kaplan turbine

$\because N_s \propto \frac{1}{H^{5/4}}$

The turbine with low specific speed will have higher head. Eg. Pelton turbine

5. Efficiency

The turbine with highest efficiency under various working condition shall be selected.

6. Maintenance Cost

The maintenance cost of reaction turbine is more than that of impulse turbine.

7. Transport Consideration

In case of larger units, it may be difficult to transport assembled large sized runner to the powerhouse sites.

8. Disposition of Shaft

From previous experience, it is recommended that horizontal shaft arrangement is best suitable for large size impulse turbine eg. Pelton turbine where as vertical shaft arrangement is most suitable for large sized reaction turbine eg. Francis trubine.

9. Cavitation Characteristics

Cavitation characteristics affects the installation of reaction turbine.

10. Water Quality

Water quality is more crucial for reactive turbine than reaction turbine.

Types of Energy Dissipators in Hydraulic Structure

Energy dissipater is a structure provided behind an overflow section, for example  a spillway, in order to dissipate the excess kinetic energy of water at downstream of the spillway. A spillway provided in the dam site always consists of an energy dissipating structure at the the toe of the dam. It kills the excess energy of surplus water and thus prevents damages to the dam and any other appurtenant structures in the downstream. Energy dissipation of water passing over the crest of spillway may be achieved by one of the following methods:

1. Hydraulic Jump Type Stilling Basin
2. Roller Bucket
3. Deflector Bucket / Flip Bucket / Ski Jump Bucket / Trajectory Bucket

1. Hydraulic Jump Type Stilling Basin

An stilling basin is a structure provided at the toe of spillway in order to dissipate the energy of excess water coming from spillway by formation of hydraulic jump within the confines of the basin. The flow passing at critical depth over the crest of spillway becomes super critical at dam toe which when meets the normal flow at subcritical depth in the downstream side, a hydraulic jump is formed during the transition from super critical to subcritical flow. The stilling basin should be so designed that the hydraulic jump is formed within the length of the basin so as to not harm the channel behind the length of stilling basin. In order to achieve this, the post jump depth obtained from sequent depth relation should be exactly equal to tail water depth. A stilling basin consists of a concrete apron and some auxiliary structures such as end sill, chute blocks, baffle blocks, etc.

2. Roller Bucket

Roller bucket is used to dissipate the energy in situation when the tail water depth is much more than the post jump depth. When high velocity sheet of water slides down the spillway, it gets arrested by the tail water. As a result, excess energy is dissipated due to formation of submerged hydraulic jump. A roller may be either a solid roller bucket or a slotted roller bucket the latter being the improved version of the former. The bucket type energy dissipater has a relatively short length as compared to hydraulic jump type stilling basin. The major design parameters for a roller bucket are the radius of bucket (R) and lip angle $(\phi)$. The radius varies from 15 to 25 m and lip angle varies from 20° to 40°.

3. Deflector Bucket / Flip Bucket / Ski Jump Bucket / Trajectory Bucket

Deflector bucket is used to dissipate the energy in situation when the tail water depth is insufficient for the formation of hydraulic jump. i.e. tail water depth is much less than post jump depth. It is in construction very similar to roller bucket but the hydraulic action is entirely different. The trajectory bucket deflects the high velocity jet into the air and is made to strike the river bed at a considerable distance from the structure. This type of energy dissipater is suitable for the situation where foundation rock is of good quality and can withstand the erosive action of striking jet. The energy dissipation is achieved due to combined action of air resistance, viscous effect and turbulence due to impact on the river bed.

Classification of Hydropower Plants | Types of Hydropower Projects

Classification of Hydropower Plants

a) Based on Purpose

1. Single Purpose Project

It is solely designed for the purpose of hydroelectricity generation.

Eg: Upper Tamakoshi Hydropower Project

       Khimti Khola Hydropower Project

2. Multipurpose Project

It is designed to fulfill more than one function or objectives. For example, the water diverted for hydroelectricity generation may also be utilized for irrigation purpose.

Eg: Bheri Babai Diversion Multipurpose Project

      Sunkoshi Marin Diversion Multipurpose Project

b) Based on Operation

1. Isolated Plant

Micro and mini hydropower plants in rural areas may be designed to serve particular village only and is not connected to the national grid is called isloated plant.

2. Grid Connected Plant

Hydropower plant with a power station feeding to a grid is called grid connected plant.

c) Based on Head

According to Dandaker and Sharma, hydropower plants can be categorized based on head as below:

Low head plants: <15m

Medium head plants: 15 - 70m

High head plants: 71 - 250m

Very high head plants: >250m

In case of Nepal, the following classification can be adopted:

Very high head plant: >350m

High head plant: 150 - 350m

Medium head plant: 60 - 150m

Low head plant: Below 60m

Very low head plant: Upto 15m

d) Based on Plant Capacity

As per Dandaker and Sharma, hydropower plants can be categorized based on capacity as below:

Micro hydel plants: <5 MW

Medium capacity plants: 5 - 100 MW

High capacity plants: 101 - 1000 MW

Super plants: >1000 MW

In case of Nepal, the following classification can be adopted:

Micro plants: upto 100 KW

Mini plants: 100 - 1000 KW

Small plants: 1 - 25 MW

Medium plants: 25 - 100 MW

Large plants: >100 MW

e) Based on Storage Capacity

1. Run of River (ROR) Project 

Those plants which do not regulate the hydrograph of source river in seasonal term, are known as ROR plants. Such plants are located in perennial river. Weir is constructed across the river to maintain the required water level u/s of weir and water is diverted into a waterway. It may have following three possible layouts:

• ROR project with canal system
• ROR project with pipe option
• ROR project with tunnel option

Keeping the considerations during peak hours, ROR plants may be constructed with pondage, which can regulate daily hydrograph or weekly hydrograph and store water (full or partial) to run the plant under full capacity is called PROR plant.

 Fig: General Layout of ROR type hydropower project

2. Storage Project

Those plants which can regulate the hydrograph of river by one or more seasons, are usually known as storage plants. Such plants are located in non-perennial rivers. A dam is constructed across the river that creates a large reservoir in front of it. It may be of following types:

• Storage project with powerhouse at dam toe
• Storage project with powerhouse at certain distance d/s of dam

The storage project may be of seasonal storage, annual storage, and pumped storage based on regulation of water. Pumped storage plants use excess electricity during periods of low demand to pump water from a lower reservoir to an upper reservoir. Then, during periods of high electricity demand, the stored water is released from the upper reservoir to the lower reservoir, generating electricity in the process.

What is Cavitation & its Effects in Turbine ?

What is Cavitation ?

Cavitation is a phenomenon that arises when the pressure of a liquid drops below its vapor pressure, causing the formation of vapor bubbles or cavities. Pressure drop may occur in the region of high flow velocities, for eg. at the exit of turbine runner. As the water flows through the turbine, its velocity increases. And according to Bernoulli's principle, an increase in flow velocity causes increase in velocity head and hence decrease in  pressure head since the total head always remains constant.

What Causes Cavitation in Turbine ?

When prevailing pressure falls towards vapour pressure of liquid, water starts vaporising and at the same time, normally dissolved gas gets liberated due to low ambient pressure. The water vapour and the liberated gas thus forms minute microscopic bubbles in the flowing water. When these bubbles get transported to the zone of higher pressure which is high enough to overcome the surface tension of bubbles, they get collapsed. When millions of such bubbles collapse simultaneously, a shock wave similar to water hammer but of short duration is produced which slowly causes erosion of concrete and metal surfaces.

What are Harmful Effects of Cavitation in Turbine ?

• Erosion of concrete and metal surfaces.
• Vibration and noise of machine parts
• Loss of material due to pitting
• Reducing the actual volume of liquid due to formation of bubbles

How to avoid Cavitation in Turbine ?

• A careful streamlined design of flow passage of the runner and draft tube.
• The sub atmospheric pressure at runner exit should be kept resonably above the vapour pressure limit.
• By using metals more resistance to cavitation damage.
• By periodic inspection and maintenance of turbine.

Working Principle & Functions of Draft Tube in Turbine

Draft tube is a pipe of gradually increasing cross-section that connects the outlet of turbine runner to the tailrace. It is used for discharging the water from exit of a reaction turbine to the tail pool and is provided only for reaction turbines eg. Francis turbine. Its cross section gradually expands and also changes its shape along its length from circular at inlet to the rectangular at the end. A draft tube plays an important role in optimizing the performance and efficiency of turbine.

Working Principle of Draft Tube

Applying Bernoulli's equation between runner exit (1-1) and draft tube outlet (2-2):

$$z_1+\frac{p_1}{\gamma}+\frac{v_1^2}{2g}=z_2+\frac{p_2}{\gamma}+\frac{v_2^2}{2g}+h_f$$

$$or, (H_s+h)+\frac{p_1}{\gamma}+\frac{v_1^2}{2g}=(\frac{p_{atm}}{\gamma}+h)+\frac{v_2^2}{2g}+h_f$$

$$or, \frac{p_1}{\gamma}=\frac{p_{atm}}{\gamma}-H_s-\frac{v_1^2-v_2^2}{2g}+h_f$$

$$or, \frac{p_1}{\gamma}=\frac{p_{atm}}{\gamma}-(H_s+\frac{v_1^2-v_2^2}{2g})+h_f \tag{1}$$

where,

$H_s$ = Static suction head

$\frac{v_1^2-v_2^2}{2g}$ = Dynamic suction head

$h_f = k\frac{v_1^2-v_2^2}{2g}$

$$or, \frac{p_1}{\gamma}=\frac{p_{atm}}{\gamma}-(H_s+\frac{v_1^2-v_2^2}{2g}-h_f )$$

$$or, \frac{p_1}{\gamma}=\frac{p_{atm}}{\gamma}-\left[H_s+(1-k)\frac{v_1^2-v_2^2}{2g}\right]\tag{2}$$

Now, draft tube efficiency can be written as:

$${\eta}_d=\frac{\text{Actual regain of pressure head}}{\text{Velocity head at entrance of draft tube}}$$

$$ =\frac{v_1^2-v_2^2}{2g}-h_f$$

$$=(1-k)\frac{v_1^2-v_2^2}{2g}$$

$$\therefore {\eta}_d=\frac{\frac{v_1^2-v_2^2}{2g}-h_f}{\frac{v_1^2}{2g}}$$

$$\therefore {\eta}_d=\frac{(1-k)\frac{v_1^2-v_2^2}{2g}}{\frac{v_1^2}{2g}}$$

From equation (2), it is clearly known that there exists a negative pressure at runner exit which is equal to $H_s+(1-k)\frac{v_1^2-v_2^2}{2g}$. From this, following two conclusions can be drawn:

1. Due to the use of draft tube, the turbine will not lose head $H_s$ becasue of equal reduction in pressure head at runner exit.
2. Due to use of draft tube of increasing cross-section, the pressure value at runner exit further reduced by $(1-k)\frac{v_1^2-v_2^2}{2g}$.

Purpose / Function of Draft Tube

1. It helps to achieve the recovery of velocity head at runner outlet which otherwise would have gone to waste as an exit loss.
2. It allows the turbine to be set at higher elevation without losing advantage of elevation difference.
3. It serves as a passage for water from runner exit to tail pool.

Factors Affecting Selection of Foundation

Selection of particular type of building foundation is affected by various factors which are explained below:

1. Type of soil

Shallow foundation are preferred if the soil close to the surface has good bearing capacity. If the soil is not capable of supporting structural loads then deep foundation are required.

2. Load from Superstructure

If the structural loading is relatively small, shallow foundation may withstand load from superstructure. In case of high rise building with intense loading, deep foundations may become the only choice.

4. Settlement

If the foundation settlement is not within the allowable limit, then choice of foundation type may vary accordingly.

5. Property Line

Due to restriction of property line, a column may have to be placed at the edge of footing creating an eccentricity. In such case, a cantilever footing (or strap footing) should be provided.

6. Stress Overlap

If the spacing between column is very small, then the stress from independent footings might overlap and become larger than allowable limit. Thus, combined footing have to be preferred.

7. Local Building Codes & Regulations

Building codes and regulations set by local authorities dictate the minimum standards and requirements for foundation design and construction. Compliance with these regulations is essential to ensure the safety and stability of the building.

8. Environmental Factors

Environmental conditions, such as seismic activity, flooding etc need to be considered when selecting a foundation system. Regions prone to earthquakes, for example, may require specialized foundation designs to withstand the seismic forces.

9. Type of Structure in Neighborhood

High rise buildings may cause uplift of nearby building due to soil heaving. So, a pile foundation may be the solution to safely transfer load to the deep strata and not to harm the nearby structures if any.

10. Other Factors

• Construction cost and time
• Service life of structure
• Safety Margin
• Ground water table
• Site topography
• Depth of hard strata