Shear stress in a turbulent flow is due to the fluctuation of velocity in the direction of flow as well as transverse to it. Let's understand this in detail:

1. Introduction:
- Shear stress is a force that acts parallel to a surface, causing one layer of fluid to slide over an adjacent layer.
- Turbulent flow is characterized by chaotic and irregular fluid motion, with fluctuations in velocity occurring in various directions.
- Understanding the causes of shear stress in turbulent flows is essential in various engineering applications, such as pipe flow, boundary layers, and mixing processes.

2. Viscous Property of the Fluid:
- The viscous property of a fluid refers to its resistance to shear deformation.
- In laminar flow, where fluid motion is smooth and ordered, shear stress is primarily due to the viscous property of the fluid.
- However, in turbulent flow, the role of viscosity becomes less significant compared to the effects of fluctuating velocities.

3. Fluctuation of Velocity:
- Turbulent flow is characterized by eddies, swirls, and vortices that continuously form and break down.
- These turbulent structures cause velocity fluctuations in both the direction of flow and transverse to it.
- The fluctuations in velocity result in different layers of fluid moving at different speeds and in different directions.
- As a result, shear stress is generated at the interface between adjacent fluid layers, causing them to slide over each other.

4. Turbulent Energy Cascade:
- Turbulent flows are driven by the transfer of energy from large-scale structures to smaller scales.
- This energy cascade leads to the formation of smaller and smaller eddies, increasing the intensity of velocity fluctuations.
- The continuous generation and dissipation of these eddies contribute to the generation of shear stress in turbulent flows.

5. Implications and Applications:
- Understanding shear stress in turbulent flows is crucial in various engineering disciplines, such as fluid dynamics, heat transfer, and chemical engineering.
- It affects the design and analysis of systems involving fluid flow, such as pipes, channels, and aerodynamic surfaces.
- Accurate prediction and control of shear stress in turbulent flows can help optimize the efficiency and performance of these systems.

In conclusion, shear stress in a turbulent flow is primarily due to the fluctuation of velocity in the direction of flow as well as transverse to it. This phenomenon arises from the chaotic and irregular motion of the fluid, leading to the formation and breakdown of turbulent structures. Understanding and quantifying shear stress in turbulent flows is essential for various engineering applications.