The decrease in drag force magnitude as speed increases in viscous flow can be attributed to the influence of two primary factors: the thinning of the boundary layer and the transition from laminar to turbulent flow.
In viscous flow, such as the flow of a fluid past an object, a boundary layer forms near the surface of the object. The boundary layer is a region of fluid where the effects of viscosity are significant. It can be further divided into two layers: the laminar sub-layer close to the surface and the turbulent outer layer.
At lower speeds, the boundary layer tends to be thicker, and the flow within it is predominantly laminar. In laminar flow, the fluid moves in smooth layers with little mixing or disruption. This laminar flow creates a relatively high drag force because the fluid near the object's surface is dragged along due to viscous effects.
However, as the speed increases, the boundary layer starts to thin. The thinner boundary layer reduces the area of fluid that experiences significant viscous effects, leading to a decrease in drag force. This thinning occurs because the faster-moving fluid outside the boundary layer transfers momentum to the slower-moving fluid near the surface, causing it to accelerate and reduce the thickness of the boundary layer.
Additionally, at higher speeds, the flow may undergo a transition from laminar to turbulent flow. Turbulent flow is characterized by chaotic fluid motion, mixing, and eddies. Turbulent flow has a higher capacity to entrain fluid from outside the boundary layer, leading to a more efficient transfer of momentum. This enhanced momentum transfer reduces the drag force compared to laminar flow.
Overall, the combined effects of thinning the boundary layer and the transition to turbulent flow result in a decrease in drag force magnitude as speed increases in viscous flow.