# Calculating drag and pressure forces from numerical simulations

• Janko1
In summary, the mathematician is trying to calculate the pressure and drag forces on an obstacle, but has trouble with which velocities to take. He is guessing that he has to take pressure just before the obstacle and just after the obstacle and the velocities above the obstacle at the beginning and at the end. He is also trying to use LaTex to provide the equation for the drag force. However, he is not sure how accurate this will be.
Janko1
Homework Statement
I am simulating flow around a square and have obtained pressure and velocity in whole domain. I have to calculate the drag and pressure forces, but have trouble with choosing where to take the velocities and pressures.
Relevant Equations
$$F_d = 1/2 c_p A \rho u^2$$, $$F = \dfrac{\rho A u_2 \delta t u_2 - \rho A u_1 \delta t u_1}{\delta t}$$
So as stated, I am calculating the pressure and drag forces on an obstacle, but have trouble with which velocities to take. This is my geometry: http://shrani.si/f/3l/13P/2Tihb3iM/projekt2.png . I am guessing that I have to take pressure just before the obstacle and just after the obstacle and the velocities above the obstacle at the beginning and at the end. Is this the right thinking?

Can you please use LaTex to provide the equation for the drag force. In its current form, it is unreadable.

You need to integrate the pressure over the area of the leading and trailing faces of your object to get the pressure force.

The drag shear stress on the upper face is given by ##\eta\frac{dv_x}{dy}##

Yeah, sorry.
$$F_d = 1/2 c_p A \rho u^2$$ is the drag force, and $$F = \dfrac{\rho A u_2 \delta t u_2 - \rho A u_1 \delta t u_1}{\delta t}$$ is the conservation of momentum.

So I should just take this shear stress at the upper obstacle boundary or am I missing anything else? I believe that every other force is symmetrical and cancels out?

I can see what you are doing with your 2nd equation above. You are using a macroscopic momentum balance (with a control volume) to get the force exerted by the object on the fluid, which is minus the force exerted by the fluid on the object. For the upstream end of the control volume, it is OK to just multiply by the overall area because the velocity is uniform, but, for the downstream end, you need to integrate over the area. (And, of course, cancel out those delta t's). I'm not sure how accurate an answer you will get using this approach, because I would expect lots of roundoff in the calculation.

For the pressure component of the overall force Fp, you would integrate the pressure over the upstream and downstream faces of the object, and take the difference. The frictional drag force could then be determined from the difference between F and Fp. But, like I said, I don't know how accurate it will be to get the overall force from the macroscopic momentum balance. Therefore, I would also integrate the shear stress over the upper and lower faces of the object to get the frictional drag by a second independent means.

Thank you very much for all the help, the things are clearer now and I think I will be able to properly solve the problem.

## 1. How do you calculate drag and pressure forces from numerical simulations?

Drag and pressure forces can be calculated using the Navier-Stokes equations, which describe the motion of fluids. These equations can be solved numerically using computational fluid dynamics (CFD) techniques. The resulting simulation provides information on the flow velocity and pressure distribution, which can be used to calculate the drag and pressure forces.

## 2. What factors affect the accuracy of drag and pressure force calculations from numerical simulations?

The accuracy of these calculations can be affected by several factors, including the resolution of the simulation, the complexity of the fluid flow, and the accuracy of the boundary conditions and fluid properties used in the simulation. Additionally, the choice of numerical method and the convergence criteria can also have an impact on the accuracy of the results.

## 3. Can drag and pressure forces be calculated for any type of fluid flow?

Yes, drag and pressure forces can be calculated for any type of fluid flow, including laminar and turbulent flows. However, the accuracy and complexity of the calculations may vary depending on the type of flow being simulated.

## 4. How can drag and pressure forces be visualized from numerical simulations?

Drag and pressure forces can be visualized using post-processing techniques such as streamlines, contour plots, and vector plots. These techniques allow for a better understanding of the distribution and magnitude of these forces in the simulated flow.

## 5. What are some real-world applications of calculating drag and pressure forces from numerical simulations?

The ability to calculate drag and pressure forces from numerical simulations has many practical applications, including in the design of aerodynamic vehicles such as airplanes and cars. It is also used in the design of buildings and structures to ensure they can withstand wind and water pressure. In addition, these calculations are important in the development of renewable energy technologies such as wind turbines and hydroelectric power plants.

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