External flows
External flows in general can be classified as flows over immersed bodies where flow features (such as boundary layers) around the body can develop freely without constraints of adjacent surfaces:
‐Airflow over aircraft and ground vehicles
‐Water flow over a submersible vessel
‐Flying baseballs, footballs and golf balls
?One of the most important areas of interest in studying external flows are fluid forces acting on the body, as they define the dynamics of an object traveling through a fluid.
? In this section, we will cover realistic external fluid flows across all fluid regimes from laminar to turbulent.
Examples of External Flows
Laminar vs Turbulent Flow
? The vast majority of practical external flows are turbulent in a sense that the flow is turbulent in a part of the domain around the flow.
? All turbulent external flows invariably have laminar regions, e.g., a boundary layer immediately after its onset or slow moving separation regions.
? If laminar regions are very small compared to the turbulent region, then the flow can be assumed to be fully turbulent.
? On the other hand, if a laminar boundary layer persists over a significant portion of the surface, then both laminar and turbulent regimes must be included in the analysis.
?External flows develop whenever an object is moving through a fluid.
Therefore, for external flow analysis, our primary objectives are prediction of fluid forces acting on the object and understanding the flow physics defining these forces.
Effect of Reynolds Number
? At moderate Reynolds numbers (1 < ???? < 103), the flow begins to separate in a periodic fashion in the form of Karman vortices Street.
Surface Roughness
Surface roughness affects the boundary layer by
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?Increasing the turbulent shear stress
? Reducing the distance to transition to a turbulent boundary layer
‐This causes an increase in drag in streamlined bodies (e g airplane wing) and a decrease in drag in blunt bodies (e g golf ball)
Reattachment of Separated Flow
?In some situations separated flow can reattach itself to the surface
?Very little theory exists to describe the reattachment phenomenon In broad terms it can be explained as follows
‐Separation is induced by adverse pressure gradients If a favorable pressure gradient is recovered past the separation point, then the flow can, in principal reattach itself
?A favorable gradient alone may not always be enough for reattachment.
‐If a separated laminar flow transitions to turbulent, then it has a better chance of reattachment as turbulent flows are less susceptible to separation under adverse
gradients than laminar flows due to the orders of magnitude better wall normal momentum transport.
Wakes
?Wakes are formed when the boundary layer flow departs a wall and merges into a uniform environment, for example the vortex dominated flow behind a bluff body
?A wake behind an object can be viewed as a jet “carved out” of the mainstream Just downstream of the body the wake is still developing and non similar, but 3 4 body lengths downstream it develops into a self similar form.