Flow Design focuses on flow that is slower than the speed of sound. This is called subsonic flow, and is typical in automotive, consumer goods, architectural, and many aircraft flow applications. Flow that is faster than the speed of sound is called supersonic. Examples of supersonic flow include flow around missiles and high-speed combat aircraft. Flow Design does not solve for supersonic flows.
In this topic, we discuss many of the flow patterns you may see in your wind tunnel flows. We use automotive examples to illustrate the patterns, but these principles apply to a wide range of wind tunnel flow applications.
Upstream
In the region upstream of the model, the air flow is uniform, and moves at the specified flow speed.
Impingement
The point where the flow first hits the model is called the impingement point. This is where the flow stops as it contacts the leading edge, and is where the flow splits above and below the model. The pressure is often higher here due to loading effect of the wind.
Wind Loading
For 3D flows, you can visualize the wind loading on the model surfaces by displaying Surface Pressure. A significant source of wind resistance often occurs at the front face of the model. A large, flat face resists the wind much more than a low, stream-lined design. Reducing the drag due to wind resistance is important for fuel economy and efficient operation.
Flow over the Model
As the flow passes over and around the model, the flow often accelerates because the air is constricted by the sides of the wind tunnel. For this reason, it is important to use an appropriately sized wind tunnel. If the wind tunnel is too small, the flow is artificially "pinched" as it passes around the model, which can reduce the accuracy of the results.
Flow Separation
Separation occurs when the flow stops following the shape of the object. In many cases the flow moves around in a circular pattern. Other times the flow simply detaches from the model. These regions often occur in the wake, but can also occur downstream of any protruding object. The smoother the flow, the less drag forces you'll encounter, which translates to improved energy efficiency.
Local effects such as separation off a leading edge can also lead to flow separation off the model. In the following image, the flow separates away from the car near the intersection of the hood and the windshield.
Wake
The region just downstream of the model is called the wake. This is where the flow comes back together after it passes over and around the model. The flow is usually slower in the wake, and can be chaotic. To get a better view of the flow, display vectors or flow lines. Another way to view the wake is to display pressure.
The pressure in the wake is usually pretty low, and a large wake area can significantly affect how well an object moves through air. This wake often acts as a resistance, and can reduce fuel efficiency. Designs that produce smaller and smoother wakes usually have less drag, and in many cases, are more fuel efficient.
A large wake downstream of the object means higher drag. To minimize the wake, the object shape should allow the flow to follow it as smoothly as possible. Downstream chaotic recirculation patterns often create low pressure regions which slow the object, resulting in energy-robbing drag.
Vortex Shedding and Noise
In some wind tunnel flow applications, the flow downstream of the model oscillates up and down in a repetitive pattern. This is called vortex shedding, and is common in aerodynamics. Vortex shedding can cause noise, and often affects how the flow interacts with downstream objects. A common example is when driving behind a fast-moving tractor-trailer, you'll feel a cross-wind on the front on your vehicle, alternating side-to-side. As soon as you pass the truck, the cross-wind stops. Those oscillating cross winds are the vorticies alternating off the tail end of the truck.
Noise is caused whenever air moves over a stationary object (or when an object moves through stationary air). While aerodynamic-induced noise cannot be eliminated, it can be managed within a design. Noise often occurs locally in areas that have rapid, high value oscillations. Display Velocity to identify recirculation regions and transient vortices downstream of anything that protrudes from the model.
