An experimental study is conducted to determine the characteristics of frictional pressure drops of fluid flow in porous beds packed with non-spherical particles. The objective is to examine the applicability of the Ergun equation to flow resistance assessment for packed beds with non-spherical particles. The experiments are carried out on the POMECO-FL facility at KTH. Hollow spheres and cylinders are used to pack the beds. Either water or air is chosen as the working fluid. The experimental data show that the Ergun equation is applicable to all the test beds if the effective particle diameter used in the equation is chosen as the equivalent diameter of the particles, which is the product of Sauter mean diameter and shape factor of the particles in each bed.Keywords Porous media · Non-spherical particles · Frictional pressure drop · Equivalent diameter List of SymbolsA, B, C Constant A p Surface area of particle, m 2 A sp Surface area of the equivalent-volume sphere, m 2 d Particle diameter, m d e Effective diameter, m d eq Equivalent diameter of non-spherical particles, m d p Particle diameter, m d sd Sauter mean diameter, m d vs Volume-surface mean diameter, m d t Tube diameter, m 123 36 L. Li, W. Ma J Superficial fluid velocity, m/s K Permeability L Beds length, m M Mass of particles, kg P Pressure drop, kPa S V Specific surface area of particles, m −1 V p Volume of particles, m 3 V 0Total volume of the porous bed occupied, m 3

Background Management of cranial defects following failed cranioplasty due to titanium mesh exposure and infection is challenging. The purpose of this report is to describe a modified technique using a free myocutaneous flap transfer for primary soft tissue reconstruction, and titanium mesh reinsertion for cranioplasty revision. Methods Nineteen patients with titanium mesh exposure and infection following cranioplasty were treated from January 2012 to January 2019. The average patient age was 41.89 years and the average size of the cranial defect was 7.74 × 13.92 cm. The reasons for craniotomy were craniocerebral trauma (n = 17), cerebrovascular disease (n = 1), and brain tumor (n = 1). The mean duration between implant exposure and current procedure was 7.16 months. Implant was removed and a free myocutaneous flap was designed to cover both scalp and cranium defects. After a mean duration of 12.32 months, implants were re‐inserted in a vascularized pocket at the second stage by elevating a plane between the previously transferred fascia layer and muscle layer. Results The average sizes of the muscle flaps and skin paddles were 7.74 × 13.92 cm and 4.97 × 8.97 cm. The average size of the implants was 8.24 × 14.42 cm. All flaps survived completely with no complication. After an average follow‐up of 48.16 months there were no cranioplasty failures. Functional coverage of craniectomy defect sites with normalized head contour was achieved. Conclusions The use of free myocutaneous flap and implant reinsertion achieved durable cranial and scalp defect reconstruction and aesthetic outcomes. The myocutaneous flap increases blood supply to the scalp, which may reduce the chances of infection and implant re‐exposure.

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