3D simulation of flow passing through symmetric bodies and nose optimization of these bodies to achieve the lowest drag

Master's Thesis in Mechanical Engineering - Energy Conversion

One ??of the ways to reduce energy consumption for underwater devices is to reduce the drag on these devices. The nose of underwater objects is one of the most important parts of these objects in dealing with currents. By optimizing this part, the drag can be reduced by controlling the boundary layer of the fluid, by reducing the turbulence of the flow and even preventing the formation of turbulence in the boundary layer. In this thesis, in order to achieve the best possible nose, a mathematical formula has been used to cover all possible curves and the best curve with the least drag is selected from among these curves. Then we compare the drag obtained from the optimal mode with the model available from the laboratory and reach interesting results in this field. In this investigation, simulation based on computational fluid mechanics has been done for a model with a zero degree angle that has speed. SST K-? turbulence model has been used to simulate the turbulent flow. At the end, a comparison was made with different turbulence models and the obtained drag value was compared. It should be noted that in this optimization, the effects of the blades that exist in the tail of these devices and are used to create thrust have not been seen.

Keywords: Symmetric bodies, turbulence model, drag coefficient, computational fluid dynamics

1-1-Introduction

Fluid flow plays an important role in the industries around us such as turbomachines, hydrodynamic systems, air and space industries, oil and gas industries and many more. Since in most industries and systems, the flow regime is turbulent, so this type of flow is extremely important. The reason for its importance is that the turbulent flow plays an important role in the transfer of momentum, heat and mass transfer, energy loss and friction in fluid systems. Therefore, in order to optimally and optimally design fluid systems in various industries, it is necessary to know turbulent flows and determine their parameters. Determining these parameters is done by numerical and experimental methods. In numerical methods, by using simulation and solving equations governing fluid flow, such as continuity, motion and energy equations, flow parameters are obtained in different conditions and according to the obtained results, desired systems are designed or optimized. in experimental methods using equipment such as wind tunnel, water tunnel and The model is placed in the test conditions and by using measuring devices, different fluid flow parameters are measured, as a result, physical phenomena can be understood and fluid systems can be designed and optimized. The above two methods have their respective advantages and disadvantages that researchers and designers should use the advantages of both methods optimally.

In experimental methods, models, test equipment and measuring devices are needed and are usually more expensive than numerical methods. Due to the problems of measuring some parameters of fluid flow or unsteady flows in very short times, such as checking the flow around an aerodynamic object from zero moment to the time of boundary layer formation, it is very complicated and difficult to use experimental methods. In numerical methods, the equations governing fluid flow are solved by different methods. In these methods, due to the simplification of the equations governing the fluid flow, the error caused by the turbulence model or the influence of the boundary conditions, there is a possibility of error in the obtained results, which is better to compare the accuracy of the results with the results obtained from experimental methods and correct the written codes. Currently, according to research costs, it is better to use both numerical and experimental methods in a complementary manner [1]. For example, in structural engineering, to determine the way of loading from wind force or to know the flow of air around structures such as buildings, bridges, stadiums, etc.. It is necessary to determine the pressure distribution, velocity distribution, spectrum of air flow disturbances and the thickness of the boundary layer of the air flow. In order to check and measure these parameters, it is necessary to conduct an experiment, in this way, a small model of the desired structure is made and the behavior of the air flow around the model is checked using a wind tunnel. What is important in this method. Placing the model inside the boundary layer and creating a geometric and dynamic similarity between the air flow inside the wind tunnel and the atmospheric flow. This is done by parameters such as the Reynolds number, the way the velocity is distributed around the model, and measuring the spectrum of air flow disturbances. In order to investigate the vibration behavior of structures, it is very important to measure the type of frequency of air flow disturbances. Therefore, it can be seen that the accurate measurement of air flow parameters around the model is very important and any mistake in the measured values ??can cause a mistake in the design. One of the important fluid flow parameters is the instantaneous velocity of the fluid flow. The instantaneous speed of the fluid flow can be shown as a vector that has components W(t), V(t), U(t) respectively in the Cartesian coordinates. Instantaneous speed at a point can be shown as the sum of average speed and speed disturbances:

Equation 1. Speed ??equations

Measuring the components of the disturbance and their changes in the domain of time or frequency is of special importance in knowing the fluid flow and controlling it. The frequency of fluid flow velocity disturbances varies from a few Hz in smooth flow to several kHz in turbulent flow and depends on the Reynolds number. Also, the mutual effect of u and v on each other is very important.

In experimental methods, fluid flow velocity determination is done in two ways, direct and indirect. In the indirect method, the speed of fluid flow is measured by measuring pressure and using the laws of fluid mechanics, and in the direct method, using devices such as laser flowmeters, hot wire flowmeters, etc. is measured In the direct method, the output of the hot wire flow meter device, which is usually in the form of voltage, must be calibrated first, then the fluid flow velocity is measured using the output voltage and calibration equations. In the indirect method, the dynamic pressure of the fluid flow is measured using the pitot static tube, and the average velocity of the fluid flow is determined using the laws of fluid mechanics. In this method, fluid flow disturbances cannot be measured. On the other hand, the frequency response of pressure measuring devices is not high, and using this method, it is only possible to measure the instantaneous speed with a frequency of several hundred cycles per second. To measure high-frequency instantaneous velocities, as well as when the rapid response of the measuring device to fluid flow changes is considered, a hot wire flow meter or a laser flow meter is used. The hot wire flow meter is an instrument that can measure the instantaneous velocity of the fluid flow with a very high frequency and using the measured instantaneous velocity, average velocity, fluid flow disturbances, Reynolds stresses, flow angle (if two or three dimensional hot wire is used), flow direction (especially in reverse flows), two phase flow parameters can be measured.

The basis of the hot wire flow meter, Heat transfer is from a hot wire with a very low diameter (about a few micrometers) made of tungsten, platinum or platinum alloys. This hot wire is installed on two bases and placed in the path of fluid flow. Any change in fluid flow conditions that affects the rate of heat transfer from the wire is determined by the hot wire flow meter. It is worth mentioning that when the speed of the fluid flow decreases, the sensitivity of other methods to the change of flow conditions decreases. But the sensitivity of the hot wire current meter increases with decreasing speed. Therefore, it is better to use a hot wire flow meter to measure and study fluid flow at low speeds [2]. these devices.