<title>Active stability control of shape memory tubes conveying fluids</title>

Abstract

The instability of elastic tubes conveying fluids is controlled by manufacturing these tubes from a NIckel-TItanium alloy (NITINOL) which has unique shape memory characteristics. For example, the modulus of elasticity of the NITINOL tubes increases about threefold as the alloy undergoes its martensitic phase transformation. Deflection of the tube off the unbuckled shape, due to high internal flow velocities and elastic instabilities, triggers an active control system which activates the shape memory effect to increase the flexural rigidity of the tubes and bring them back to their original undeflected shape.A mathematical model is developed to describe the fundamentals governing the operation of this class of actively controlled NITINOL tubes while conveying fluids.The model describes the interaction between the fluid flow, the elastic behavior of the tubes and the thermal activation of the shape memory effect.The predictions of the model are validated experimentally using a 0.58 m long NITINOL-55 tube that has outer diameter of 2.134 mm and inner diameter of 1.778 mm. The effect of varying the activation strategy of the shape memory effect on the critical flow velocity, at which flow-induced instabilities occur, is determined theoretically and experimentally.It is shown that the shape memory controller can significantly increase these critical flow velocities as it stiffens the tubes by virtue of the strain energy imparted by the shape recovery process. Furthermore, the effectiveness of the controller is demonstrated also by its speed of response as it brings the unstable tubes back to their undeflected shape while conveying fluids at velocities equal to their uncontrolled critical velocities. Close agreement is obtained between the theoretical predictions and the experimental results.The results presented suggest the potential of the shape memory controller as a viable means for enhancing the elastic stability of tubes conveying fluids.