This paper reviews analytical solutions for the accelerated flow of an incompressible Newtonian fluid in a pipeline. This problem can be solved in one of two ways according to the (1) imposed pressure gradient or (2) flow rate. Laminar accelerated flow solutions presented in a number of publications concern cases where the two driving mechanisms are described by simple mathematical functions: (a) impulsive change; (b) constant change; (c) ramp change, etc. The adoption of a more complex and realistic description of the pressure gradient or flow rate will be associated with a profound mathematical complexity of the final solution. This is particularly visible with the help of the universal formula derived by several researchers over the years and discussed in this paper. In addition to the solutions strictly defined for laminar flow, an interesting extension of this theory is the theory of underlying laminar flow for the analysis of turbulent accelerated pipe flows (TULF model developed by García García and Alvariño). The TULF model extends the Pai model developed more than 60 years ago, which has been previously used for steady flows only. The discussed solutions extend the theory of analytical solutions of simplified two-dimensional Navier–Stokes equations and can be used not only to study the behavior of liquids during accelerating pipe flow but they can also be used to test the accuracy of commercial CFD codes.

To simulate transient cavitating pipe flow, the discrete gas cavity model (DGCM) is combined with first-order and second-order finite-volume method (FVM) Godunov-type schemes. The earlier discrete vapor cavity model (DVCM) and DGCM based on the method of characteristics (MOC) are known to produce unrealistic pressure spikes. The new FVM-DGCM extends the previously developed FVM-DVCM through the introduction of a very small amount of free gas at the middle of each computation cell. Importantly, a pressure adjustment procedure is proposed to establish the relation between the cavity and the halves of the reach. Predictions of FVM-DGCM are compared with those of FVM-DVCM and MOC-DGCM and with experimental data. Results show that the proposed model reproduces the experimental pressure histories considerably better than the other two models. In particular, it produces fewer spikes, but—as in the old models—the first pressure peak due to cavity collapse is predicted much better than the subsequent peaks. The second-order FVM-DGCM is found to be accurate and robust, even for Courant numbers significantly less than 1.

Analytical formulas for laminar water hammer in horizontal pipes were extended and simplified into a compact mathematical form based on dimensionless parameters: dimensionless time, water hammer number, etc. Detailed treatment of turbulent water hammer analytical solutions is beyond the scope of this paper. In the Muto and Takahashi solution, novel Laplace and time domain formulas for flow velocity and wall shear stress were developed. A series of comparative studies of unified analytical solutions with numerical solutions and the results of measurements were carried out. The study shows that models that account for the frequency-dependent nature of hydraulic resistance agree very well with experimental results over a wide range of water hammer numbers Wh, particularly when Wh ≤ 0.1.

Omission of frequency-dependent hydraulic resistance (skin friction) during modelling of the water hammer phenomenon is unacceptable. This resistance plays a major role when the transient liquid flow occurs in rigid-walled pipes (steel, copper, etc.). In the literature, there are at least two different modelling approaches to skin friction. The first group consists of models based on instantaneous changes in local and convective velocity derivatives, and the second group are models based on the convolution integral and full history of the flow. To date, more popular models are those from the first group, but their use requires empirical coefficients. The second group is still undervalued, even if based on good theoretical foundations and does not require any empirical coefficients. This is undoubtedly related to the calculation complexity of the convolution integral. In this work, a new improved effective solution of this integral is further validated, which is characterised with the use of a simplified weighting function consisting of just two exponential terms. This approach speeds the numerical calculations of the basic flow parameters (pressure and velocity) significantly. Presented comparisons of calculations using the new procedure with experimental pressure runs show the usefulness of the proposed solution and prove that it maintains sufficient accuracy.

Most of today’s water supply systems are based on plastic pipes. They are characterized by the retarded strain (RS) that takes place in the walls of these pipes. The occurrence of RS increases energy losses and leads to a different form of the basic equations describing the transient pipe flow. In this paper, the RS is calculated with the use of convolution integral of the local derivative of pressure and creep function that describes the viscoelastic behavior of the pipe-wall material. The main equations of a discrete bubble cavity model (DBCM) are based on a momentum equation of two-phase vaporous cavitating flow and continuity equations written initially separately for the gas and liquid phase. In transient flows, another important source of pressure damping is skin friction. Accordingly, the wall shear stress model also required necessary modifications. The final partial derivative set of equations was solved with the use of the method of characteristics (MOC), which transforms the original set of partial differential equations (PDE) into a set of ordinary differential equations (ODE). The developed numerical solutions along with the appropriate boundary conditions formed a basis to write a computer program that was used in comparison analysis. The comparisons between computed and measured results showed that the novel modified DBCM predicts pressure and velocity waveforms including cavitation and retarded strain effects with an acceptable accuracy. It was noticed that the influence of unsteady friction on damping of pressure waves was much smaller than the influence of retarded strain.